Building-Integrated Photovoltaics (BIPV) involves the process of integrating energy (electricity) producing photovoltaic technology into residential, commercial, and industrial building and construction design and materials. By using BIPV, the solar electricity producing components actually become an integral part of the building or construction materials and design, often serving as the protective exterior weathering skin and/or interior building components. Semi-transparent or see-through solar PV modules comprise the most attractive segment of BIPV applications. These modules can be used in many applications including window glazing in building windows. In some applications they are also a part of shading devices such as car parking covers. Such BIPV systems are also known as “shadow-voltaic” systems. See-through BIPV modules can be also part of energy efficient glazing, where they are used instead of usual glass.
Currently, about 80% of the BIPV applications are served by crystalline semiconductor cell technology, while inorganic thin-film (TF) technologies account for the remaining 20% of the total BIPV market. However, the TF technologies are projected to capture over 50% of the BIPV applications by 2015. The TF technologies include amorphous semiconductor (a-Si), cadmium telluride (CdTe), copper-indium-gallium-diselenide (CIGS), and organic PV. Among them, CdTe and CIGS promise higher efficiencies than a-Si. However, these TF technologies in practice offer efficiencies in the range of 5% to 12%, with the TF see-through BIPV module efficiencies being essentially limited to the single-digit efficiency range of 4% to 8%. Both organic PV (OPV) and dye-sensitized solar cells (DSSC) are considered to be the third generation BIPV technologies (both currently providing module efficiencies on the order of 6%). All the TF and DSSC BIPV technologies currently offer much lower efficiencies than crystalline semiconductor BIPV. However, the TF technologies provide better aesthetics than crystalline semiconductor, particularly for see-through BIPV module applications. In a typical see-through crystalline semiconductor BIPV module for solar glass applications, the crystalline semiconductor cells are spaced apart to allow for visible light transmission in between the tiled cells. While this see-through crystalline semiconductor PV modules can provide relatively high effective efficiencies (e.g., typically in the range of 10% to 12%), they do not offer very attractive aesthetics, both due to the tiled design and also due to the standard busbar emitter interconnects in the cells (thus, showing visible metallization fingers and busbars).
Dye-sensitised solar cells (DSSC) operate based on the interaction between light and a dye coated onto small grains of titanium dioxide. The grains are placed in a liquid that acts as an electrolyte, collecting the electrons released by the dye as it absorbs light, thus, generating current. The whole mixture is sandwiched between a transparent glass sheet electrode doped with tin oxide to make it electrically conducting, and a rear panel. The efficiency of DSSC designed for outdoor conditions is currently about 6%. This is far below the efficiency of standard crystalline semiconductor BIPV modules.
OPV and DSSC BIPV modules cannot easily compete with the conventional crystalline semiconductor or TF BIPV solar panels due to their relatively low conversion efficiencies and shorter operational lifetimes. Crystalline semiconductor solar cells and modules have proven long lifetimes in excess of 25-30 years in the field and no TF or DSSC technology can offer or match such track record. While the conventional crystalline semiconductor wafer BIPV is only suitable for rigid BIPV applications, the TF and DSSC BIPV modules can be used for both rigid and flexible substrate applications.
This invention provides a 3D crystalline (including mono-crystalline) thin-film semiconductor substrate for making disruptive, high-efficiency, and low cost see-through solar cells. The semiconductor film thickness may be in the range of a few microns to tens of microns (up to ˜100 μm).
For example, U.S. Pat. Pub. No. 2008/0264477, U.S. Pat. Pub. No. 2008/0289684, U.S. Pat. Pub. No. 2008/0295887 and U.S. Pat. Pub. No 2009/0107545 by common inventor Mehrdad M. Moslehi disclose methods for manufacturing a 3-Dimensional Thin-Film Soar Cell (3-D TFSC). The methods comprise forming a 3-Dimensional Thin-Film Substrate (3-D TFSS) using a semiconductor template. The template structures may comprise any combination or variation of three-dimensional surface features such as a plurality of posts and a plurality of trenches between said a plurality of posts or a plurality of inverted three-dimensional pyramid surface cavities. The 3-D TFSS is formed by forming a sacrificial layer on the template, subsequently depositing a semiconductor layer, selective etching the sacrificial layer and releasing the semiconductor layer from the template. More specifically, the semiconductor layer is a self-supporting, free-standing three-dimensional (3D) epitaxial semiconductor thin film deposited on and released from a low-cost reusable crystalline semiconductor substrate template. The reusable semiconductor template may be reused to form the 3D film numerous times before being reconditioned or recycled. Select portions of the released 3-D TFSS are then doped with a first dopant, and other select portions are than doped with a second dopant. After surface passivation processes, emitter and base metallization regions are formed to complete the solar cell structure.
Known 3-D TFSS fabrication methods provides fabrication process improvements and manufacturing cost reduction by using inverted and staggered pyramid structures on the re-usable semiconductor templates. More specifically, the inverted pyramid structures disclosed in the said patent application are made by KOH-like semiconductor etching. The known crystallographic semiconductor planes on the template surface from KOH etching enable convenient epitaxial growth control as well as large number of template re-use cycles in order to amortize the template cost over numerous released cell substrates.